EP-A-1120777 discloses an optical disk with an embossed data zone which has pits and projections indicating data signals which are readable by light irradiation. A barcode pattern which indicates information is formed on the embossed data zone with a laser. To prevent copying of disk information, on a ROM-disk, physical markings are randomly formed. During the read out of the disk, the position of these physical markings is determined and compared with position information which is encrypted in the barcode. The position where the physical markings should occur is stored as a secret key in the encrypted barcode. The disk ID and the position information, which is different for different disks, are combined and encrypted together with a digital signature. Thus by comparing the actual position of the markings on the disk with the decrypted positions of the marking indicated by the barcode it is possible to determine whether the disk is a copy or not.
Both the markings and the barcode may be present in the burst cutting area, which usually is referred to as the BCA. The barcode stripes are written into the same layer as the data pits. At the reproduction apparatus, the barcode stripes are distinguished with respect to the data pits which are present in-between the barcode stripes. At the non-reflective barcode stripes, the missing portions produce a low level signal which is different from a signal generated from an ordinary data pit. A slicer is used to detect this low level. From the detected low level portions of the signal, a PWM modulator demodulates the signal containing the encrypted information.
It is a drawback that the signal generated from the data pits may disturb the correct slicing of the barcode data.